There exist a number of instances in which it would be beneficial to have a thermoset material that is able to cure and in the process build viscosity upon cure to a cured state and thereafter is amenable to reversible reduction in viscosity and subsequent rebuilding of viscosity. Such a property would be advantageous in fields as diverse as adhesives, coatings, molding, and heterogeneous encapsulation. By way of illustration, a polyurethane adhesive after thermoset cure would have a beneficial property of being able to reversibly liquefy for the purpose of either adjusting relative orientation of adhesed articles and then rebuilding viscosity and/or to replace an object adhesed to a substrate without damage to the underlying substrate. Likewise, a marred or otherwise damaged polyurethane or polyurea coating having the attribute of reversibly reducing viscosity and thereafter rebuilding to cure viscosity would be beneficial in healing such defects in a coating. In a molding application, the formation of a “lost wax” casting positive is facilitated by the ability to reduce the viscosity of a casting model while also modifying the hydrophilicity of the components from which the model is formed to facilitate mold purge prior to a mold pour. Still further, polyurethane matrices are routinely used to encapsulate heterogeneous particulate in applications such as propellants. While the polyurethane matrix is beneficial in forming shaped propellant charges and extending storage lifetime of propellant components against reaction with oxygen and/or water, at the time of propellant ignition, the viscosity of a polyurethane matrix results in a deleterious thrust profile relative to homogeneous propellants.
Diels-Alder reactions are known to produce linkages that are thermally reversible. U.S. Pat. Nos. 5,489,451 and 6,825,315 are representative of polyacrylates and polyepoxides inclusive of Diels-Alder functionalities. Unfortunately, these chemistries have not been extended to form single Diels-Alder linkages proximal to a conventional crosslinker in a polymer and under conditions of conventional crosslinking.
Thus, there exists a need for a reversible viscosity reducing polymers such as polyurea and polyurethane polymers containing Diels-Alder linkages. There further exists a need for the use of such polymers as adhesives, coatings, moldings, and polymer matrices.